1. Field of the Invention
The present invention relates to a bipolar-transistor type random access memory (RAM) device having a redundancy configuration. More particularly, the present invention relates to an emitter-coupled-logic (ECL) type RAM device having a redundancy configuration.
2. Description of the Related Art
In a RAM device, a large number of memory cells are arranged along rows and columns. The density of defects generated in such a semiconductor memory device during the manufacture thereof is relatively independent of the integration density of the device. Rather, the density of defects is derived from the semiconductor manufacturing technology. In general, the higher the integration density of the device, the greater the ratio of normal memory cells to defective memory cells. This is one of the advantages of increasing the integration density of a semiconductor memory device. Even if a device includes only one defective memory cell, however, the device cannot be operated normally, and therefore, must be scrapped. As a result, despite the lower ratio of defective memory cells, the greater the integration density the lower the manufacturing yield.
In a metal-insulator-semiconductor (MIS) type RAM, in order to overcome the problem of defective memory cells, use is made of redundancy memory cells. When a defective memory cell is detected, a redundancy memory cell row or column is selected instead of the memory cell row or column including the defective memory cell. In general, one or two redundancy memory cell rows or columns are usually provided.
In the prior art, however, no bipolar-transistor type RAMs have a redundancy configuration because there has been no redundancy configuration suitable for a bipolar-transistor type RAM. The reason is that bipolar-transistor type RAMs operates at a much higher speed than a MIS type RAM. Also, there has not been any strong demand for a redundancy configuration in a bipolar-transistor type RAM due to the comparatively small number of memory cells therein.
With improvements in integration technology for bipolar-transistor type RAMs and the requirement for larger number of memory cells, bipolar-transistor type RAM devices having a large number of memory cells have come into demand. Accordingly, bipolar-transistor type RAMs having a redundancy configuration have also come into demand.